Support and drive system for rotary grinding mills



M. TRESHOW Dem... R955 SUPPORT AND DRIVE SYSTEM FOR ROTARY GRINDING MILLS Filed May 29 1950 United States Patent SUPPORT AND DRIVE SYSTEMFOR- ROTARY GRINDING MILLS Application May 29,1950, Serial N0; 165,045 1 Claim. (Cl. 241-473 This invention relates to improvements in support and drive systems for rotatablecontai'ners such as kilns, dryers, and mills in which a charge of material is deposited to accomplish the treatment of the material during and/or by the rotation of the container;

To facilitate the discussion of my invention, it will be described as embodiediti a ball mill. However, it should be understood that the principles of the invention can be applied with equal success to other types of mills and to other types of rotating containers such as kilns and dryers. A

Ball mills of the type in which my invention is embodied are utilized to comminute various types of materials, such as cement clinkers. The comminution of the material by a ball mill is a continuous process with the untreated material being delivered to one portion of the mill and the treated material being removed from another portion thereof. Thus, the mass of the material in the mill at any one time and the balls which perform the communicating operation is substantially constant so that the mass of the entire charge (charge being defined as the material being treated and the balls accomplishing the treating) can be effectively calculated.

When the mill is rotated the charge is carried around in the mill in the direction of rotation until the surface thereof assumes an angle to the horizontal which is slightly greater than the angle of repose, the angle of repose being defined as that angle of inclination at which downward movement of the charge impends. When the charge assumes such a position, the centerof gravity of the charge is eccentrically disposed with reference to the center of gravity of the mill itself. Therefore, if the mill is driven by symmetrically arranged driving rollers, as is customary, a greater portion of the Weight of the mill and the charge therein will be imposed upon the driving rollers at one side of the mill than the other.

Such unbalance of the mill, as caused by the eccentric position of the charge, has necessitated that mills be driven by pinion gears Which mesh with girth gears dis posed about the periphery of the mill; This has been necessary because the slippage of smooth rollers against smooth surfaced girth tracks, as induced by the unbalance of the mill, has caused great power losses and inefficient mill operation, together with rapid wearing of the rollers and tracks due to such slippage. The necessity for providing girth gears which surround the cylindrical body of the mill has sharply limited the maximum size of such mills since the difiiculty of fabricating such girth gears and the expense involved in their upkeep and replacement prohibit their being utilized in i'nills having a diameter of much more than ten feet.

It is, therefore, a primaryobject of my invention to provide a support and drive system for a ball mill or similar rotating container whose component parts are so distributed with reference to said container that smooth surfaced girth tracks and smooth surfaced rollers may be used to impart rotational movement to said container.

2,726,047 Patented Dec. 6, 1955 Another object of my invention isvthe provision of drive and support means for a ball mill which includes asymmetrically disposed drive and support rollers having smooth surfaces engaging smooth surfaced girth tracks. Naturally, the utilization of such smooth surfaced rollers and girth tracks eliminates the difficulties and expense encountered in the fabrication of girth gears, and mills or" a size not previously possible become economically and mechanically feasible and desirable.

A further object of my invention is the provision of a support and drive system for a ball mill which includes first drive and support rollers which have their axes of rotation vertically spaced from a horizontal reference plane and which are disposed oppositely from second drive and support rollers which have their axes ofrotation vertically spaced a smaller distance from the same hori zontal reference plane.

Another object of my invention is the provision of a support and drive system for a ball mill which includes a plurality of leading drive and support rollers, the axes of rotation of which are spaced a substantial distance above a horizontal reference plane and a plurality of oppositely disposed trailing rollers, the axes of which are spaced a smaller distance above said horizontal reference plane than the axes of rotation of the leading rollers.

A further object of my invention is the provision of a drive and support system for a ball mill, the driveand support rollers of which are sodisp'osed with reference to opposite sides of the mill that the friction angles of the drive and support rollers on opposite sides of the mill are substantially equal at alltinies during the operation of the mill.

One of the major disadvantages of conventional mills and conventional pinion girth gear type of drives is the fact that, since a greater portion of the load is imposed by the unbalance upon the pinions at one side of the mill, unequal wear in the drive and support system for the mill occurs causing frequent breakdowns and necessitating the shutdown of the mill during the replacement of parts which have failed.

An additional object of my invention is the provision of a support and drive system for a ball mill in which the pressuresex'erted upon the drive and support rollers at both sides of the mill are substantially equal, thus equalizing the wear sustained by such rollers and eliminating the frequent breakdowns encountered in con ventional types of mills.

A further object of my invention is the provision of a ball mill which necessitates a relatively small consumption of power per unit of load because of the efiiciency with which such power is transmitted by the support and drive system associated with the mill.

Other objects and advantages of my invention will be apparent from the following specification and the accompanying drawing, which is for the purpose of illustration only, and in which:

Fig. l is a front elevational view showing a ball mill constructed in accordance with my invention; and

Fig. 2 is a diagrammatic view which serves as a basis for illustrating the manner in which the positions of the drive and support rollers of the drive and support system are calculated;

Referring to the drawings, and particularly to Fig. 1 thereof, I show a ball mill in which includes a substantially cylindrical, rotatable drum 1i. Encompassing the drum 11 is a girth track 13, the engageable surface of which is substantially smooth and which may be formed of properly shaped segments of railroad track, as shown and described in greater detail in my copending applica tion Serial No. 165,044, filed May 29, now Patent No. 2',702,2l7 of February 15, 1955.

Although only one girth track 13 is shown in Fig. 1 of the drawing, it is to be understood that a plurality of such tracks may be provided along the length of the drum 11 to provide for the support and proper rotation of the drum.

Disposed upon a horizontal surface indicated at 14 is an electric motor 15, said motor being provided with a drive sprocket 16 having entrained thereupon a link chain 17 which, in turn, engages a sprocket 18 of a gear reducer 19 which is adapted to suitably reduce the speed of rotation of the motor 15. Mounted upon the gear reducer 19 is a sprocket 21 which has entrained thereupon a drive chain 22. Rotatably mounted in a pillow block 23 is an axle 24 which has attached thereto a sprocket 25 engageable by the link chain 22 to cause the rotation of a drive and support roller 26. The surface of the drive and support roller 26 is smooth and engageable with the surface of the girth track 13 to support the drum 11 and to cause the rotation thereof. Although only one drive and support roller 26 is shown because the figure is in front elevation, it is to be understood that a plurality of such rollers may be provided and spaced along the axle 24, with the sam'' distance between the rollers as there is between the girth tracks 13 to permit the rollers to engage said girth tracks. it is to be noted that the axis of rotation of the axle 24 is vertically spaced from the horizontal reference plane indicated by the numeral 14, such vertical spacing being a criterion of the point at which the drive and support roller 26 engages the girth track 13.

Disposed at the opposite side of the drum 11 is an electric motor 28 which drives a sprocket 29 having a link chain 30 entrained thereupon, said link chain engaging a sprocket 31 of a gear reducer 32. The gear reducer 32 is provided with a drive sprocket 33 upon which is entrained a drive chain 34 which, in turn, engages a sprocket 35 mounted upon one end of an axle 36 journaled in a pillow block 37. Supported for rotation upon the axle 36 is a smooth surfaced drive and support roller 38 which engages the driven surface of the girth track 13. As previously indicated in considering the drive and support roller 26, a plurality of drive and support rollers 38 spaced along the axle 36 and supported thereby may be provided, the distance between said rollers being equivalent to the distance between the tracks which they are adapted to engage.

The axis of rotation of the axle 36 is spaced a greater distance above the horizontal plane 14 than the axle 24. Thus, the drive and support roller 26 engages the girth track 13 at a point which is closer to the vertical diameter, indicated by the line yy, of the drum 11 than the drive and support roller 38. The reasons for thus asymmetrically positioning the drive and support rollers 26 and 38 upon opposite sides of the vertical diameter y-y of the drum 11 will be presented in greater detail below.

Since the direction of rotation of the drum 11 is clockwise, as indicated by the arrow 40, the drive and support roller 38 may well be designated the leading roller While the drive and support roller 26 may well be designated the trailing roller.

As indicated in Fig. 2 of the drawing, the drum 11 provides a substantially cylindrical chamber 41 provided with a suitable wear-resistant lining 42, said chamber being adapted to receive a predetermined weight of material to be comminuted by balls 43. The balls 43 and the material, which is constantly fed and constantly removed from the chamber 41, constitute the charge 44.

As the drum 11 is rotated by the drive and support rollers 26 and 38 in a clockwise direction, as indicated by the arrow 40, the charge 44 is moved by the rotation of the drum into a position in which its surface is disposed at an angle of substantially 45 with the horizontal. At this angle, the surface of the charge is disposed at a point slightly beyond the angle of repose, the angle of repose being defined as that angle at which downward movement of the surface of the charge impends.

When so disposed, the center of gravity B of the charge 44 is positioned eccentrically with respect to the center of gravity A of the drum 11. Obviously, such eccentric positioning of the center of gravity of the charge 44 results in serious unbalance of the drum 11, such unbalance being the fact which necessitates the utilization of girth gear and pinion drives in conventional ball mills.

The manner in which the locations of the leading drive and support roller 38 and the trailing drive and support roller 26 are calculated is presented herewith in consid erable detail to facilitate the practice of the invention by those skilled in the art. The cylindrical drum 11 is rotated at a constant speed, said speed being regulatedby means of the gear reducers 19 and 32. In addition, the diameters of the leading roller 38 and the trailing roller 26 are equal to insure that the surfaces thereof will pass a given point at substantially equal speeds.

The optimum speed of the cylindrical drum 11 is usually determined by first ascertaining the critical speed of the drum. The critical speed of the drum may be defined as that speed at which particles of the material being fed continuously in the drum would be carried a full circuit around the drum during the rotation thereof; that is, the speed at which a particle of material would be carried from a point adjacent the left side of the vertical diameter y-y through the entire orbit of the drum to a point at the right side of the vertical diameter yy. Naturally, such complete circulation of the material within the drum is undesirable since it prevents the balls from operating properly upon the material to cause the comminution thereof.

It has been ascertained that the optimum speed of rotation for a drum having a diameter of ten (10) feet is approximately 70% of the critical speed, or approximately 20 R. P. M. The speed of rotation is an extremely vital factor since it determines the angle which the surface of the charge 44 assumes with the horizontal, as indicated most clearly in Fig. 2 of the drawing. Once the drum has 'reachedits optimum speed of rotation, the surface of the charge 44 remains substantially at an angle with the horizontal slightly greater than the angle of repose and the center of gravity thereof assumes a substantially constant orientation with respect to the center of gravity of the drum 11 itself.

In calculating the positions in which the drive and support rollers 38 and 26 should be disposed, the center of gravity of the drum 11 is determined to be at the point designated as A While the center of gravity of the charge 44 is determined, by a suitable calculation, to be at the point B. The centers of gravity A of the drum 11 and B of the charge 44 are then integrated to ascertain the resultant center of gravity which is located at the point C on a line drawn between the centers of gravity A and B. The resultant center of gravity C is the center of gravity of the entire mass including the drum 11 and the charge 44 as constituted by the balls and the material which is operated upon by the balls. From the position of the center of gravity C it is quite obvious that a serious unbalance of the entire mass to the left of the vertical diameter y-y of the mill exists and it is quite apparent why it has been previously impossible to utilize smooth surfaced rollers and smooth surfaced girth tracks to support and drive rotatable drums of the character here under discussion.

Having ascertained the resultant center of gravity C, the torque required to turn the mill is determined by multiplying the total weight of the mass by the distance from the center of gravity A of the drum to the resultant center of gravity C. Since it has been previously indi cated that the weight of the entire mass, including the drum 11 and the charge 44, as constituted by the balls 43 and the material being comminuted by the balls, is maintained by the continuous removal and replenishment of the material from and into the drum 11, it is obvious that the torque applied to the girth track 13 by the leading att acto s? drive: and support roller 38- and the torque applied by the trailing drive and support roller 26 must" be substan tially equally distributed on opposite sides of the center of gravity C of the entire mass to' insure the proper and efficient rotation of the drum 11.

Once the torque has been ascertained in the aboveoutlined manner, the necessary traction between the rollers 38 and 26 and the girth track 13' can be computed by dividing the torque by the radius of the track. Utilizing pounds and feet for units, the traction will be found in pounds total force for all rollers. Obviously, the necessary traction depends upon the size ofthe drum and the weight of the charge. Since the speed of rotation of the drum will be so regulated asto' maintain. the surface of the charge at a predetermined anglewith the horizontal, the necessary traction willalways be constant. It can, therefore, be said that the total necessary traction is a known factor andthe rollers must be so positioned as to obtain this traction.

It will be noted that the maximum traction which can be obtained under normal givehconditions between the rollers li and 26 and the girth track 13 is the product of the co'emcient of friction of the rollers and girth tracks and the normal pressure between the surfaces of the rollers and the girthtracks. It is obvious, therefore, that the necessary pressure upon the rollers may be deterined by dividing the necessary traction by the coefiicient of friction or a predetermined-proportion thereof. Naturally, if the rollers are placed in a very high position against the girth tracks, the Weight of the mass will create a much higher pressure on the rollers than if they were placed close together under the middle of the drum. Theoretically, if the rollers are placed close together under the middle of the drum, they willonly have to receive a pressure equal to thetotal' weight of the mass; whereas, if they are placed almost as high as the horizontal diameter xx of the drum, the pressure on each side would approach infinity.

If the rollers were positioned as high as the horizontal diameter xx of the drum; there would be pressure enough to transmit any desired amount of traction; but the pressure would be many times greater than'the weight of tne drum and it would be impossible'to make rollers and bearings which would sustain such pressure. In fact, the drum itself would be squeezed out of shape and would. be ruined by excessive side pressures.

it is, therefore, apparent that the allowable roller pressure is limited as long as the number of rollers and girth tracks must be kept Within reasonable limits determined by practical conditions and cost. Nevertheless, the necessary total minimum pressure, as computed above, must be maintained in order to operate'the mill. ifan extra high safety factor to avoid the slippage of the rollers against the girth track is chosen, a higher roller pressure will be necessary and the rollers will be placed closer to the horizontal diameter of the drum. By establishing a lower safety factor which may permit more slippage between tne rollers and the girth track and assuming a higher coefiicient of friction, the rollers would be placed lower and would receive a slighter pressure.

it will, therefore, be realized that the problem is to find positions for the rollers so that each roller can transrnit its required part of the total traction without receiving a larger pressure than necessary. This means particularly that the pressure load on each roller must be equal, and also that the traction must be divided equally between the rollers. In other words, the pressure must heapplied to all rollers in the directiondefinedby the allowable friction angle, as definedbelow. A larger friction angle would mean danger of slippage and? a. smaller friction angle would increase the danger of excessive pressures on the rollers.

To ascertain the pointsrupon the periphery ofthe-girth track at Which the drive rollers should be located, it is necessary to ascertain: the coeflicientof friction of the material embodied in the drive rollers and 'the girth track-i For purposes of exposition;- let it be assumed that the coeflicient of friction of the'g'irth track and rollers is 0:25 or 25%. To reduce the possibility of slippage between the surfaces of the rollers and the girth trackand to establish a safety fact'orof approximately 2, the coethcient of friction is divided inhalf.

It is well known that the angle of friction between any two surfaces is the angle. for which" tangent is equal to the coefficient of friction. The angle of frictionis defined as the angle betweenthe totalreaction and the normal to the surface when the limiting friction is attained. Thus, it can be seen that if the undivided coefiicient of friction were utilized to determine the position of the rollers with reference to the girth track, the rollers would be operating against the girth track at a point Where slippage between the girth track and. the rollers continuously impendecl. By dividing the methcient of friction in half to obtain a safety factor of 2, we obtain an angle which is herein designated as the friction angle, the friction angle being defined as the angle between thenormaland the resultantforce and being characterized as substantially less thanthe angle of friction; It is very necessary that the distinction between: the angle" of friction and the friction anglebe kept in mind because, if the angle of friction were used in calculating the position of the rollers, the surfaces between the rollers and the girth track would be in con-' tinual danger of slippage and the transmission of: torque and the traction between the rollers and the girth track would be very inefficient; The use of the frictionangle to determine the position of the rollers with reference to thegirth track provides a high safety factor andinsures that there will be substantially no slippage between the rollers and the girth track in normal use. By dividing the coefiici'ent of friction in halfand' obtaining the tangent thereof, we determine that the resultant isa friction angle of approximately 7.

The vertical diameter y-y has, of course; been drawn through the center of gravity A of the drum 11 and a vertical straight line is now drawn through the center of gravity C of'the entire mass, it the vertical straight line and thevertical diameter being parallel'to each other. The friction angle of approximately 7' is then plotted between the vertical diameter y-= and the vertical line drawn through the resultant center of gravity C ofthe entire mass. Since the application of force is in the direction of rotationof the drum 1-1, as indicated by'the arrow 40, thefriction angle is plotted on that side of the. vertical diameter. y-y in the direction of rotation' of the drum.

A line drawn from the point A on the friction angle intersects the vertical line through theres-ultant centerof gravity C at the point D. The next step is to plota circle, the diameter of which is the line A--D. When sopl'otted, the circumference of the circle will intersect the circumferen'ce of. the girthtrackand the p'oints of? intersection, indicated as F and G; .will be the points at which the drive and support rollers 38 and 26 should beplaced in contact with the girth track.

When the rollers-38 and 26 are disposed respectively in contactwith the girth track ISatthe points F and G; the threeforces-imposed upon the mill arid'drive assembly will be balanced; that is, the gravitational load and the driving forces imposed at F and G will neutralize each other, and the two driving forces will have' the' same numerical value and both be sodirected asto establish the predetermined friction angle with the normal and both will, therefore, bear the same pressure and contribute equally to the necessary traction with a predetermined factor of safety against slippage between the surfaces of the rollers and the girth track.

Although I have discoveredempirically that position ingttherolle'rs 38- and 26' at the points F and G respectively' will accomplish the" desirableends of equal and effective traction and permit the use of smooth surfaced girth tracks and rollers, I will present herewith proof that the tangential projections of all roller reactions are equal and turn in the same direction of rotation; that all three lines of force must intersect each other in one point; that the friction angles of the applied actual reaction forces must be equal for all rollers; and that the total pressure load on each of the rollers must be equal.

Referring to Fig. 2 of the drawing, the rollers 38 and 26 are shown with their surfaces in contact with the girth track 13 at the points F and G, respectively. As previously indicated, the rollers 38 and 26 are of equal diameters and are rotated at equal speeds by means of the gear reducers 32 and 19. The tangential forces, therefore, must be equal so long as none of the rollers slips on the girth track. In other words, the direction of force of the roller reactions must be such that the tangential projections of all roller reactions are equal and turn in the same direction of rotation.

Another condition that must prevail when the three forces, the weight of the entire mass and the roller reactions, are in balance, is that all three lines of force must intersect each other at one point.

Finally, as long as the constant speed of the drum is presumed so that all of the forces comprise a balanced system, the summation of the projections of all forces on any one line must equal zero. In the present case we will consider the projection of the forces on the horizontal diameter x-x. Since the projection of the force of gravity on such a horizontal line is zero, and since the two roller reactions come in from opposite directions and intersect at the same point, it is obvious that the horizontal projections of the wheel reactions from one side must equal the horizontal projections of the reactions from rollers at the other side.

In order to establish that the above conditions exist, the geometrical relations of the various force projections will be calculated on the basis of the diagrammatical showing of Fig. 2. First, it will be proved that the angles marked a are all equal. The line A--D connects the centers of two circles, one circle being the circumference of the girth track and the other circle being the circle inscribed with the line AD as the diameter. The two circles intersect at the points F and G which must, therefore, be located symmetrically with reference to the line AD. When straight lines are inscribed from the point A to the points F and G, these lines must form equal angles with the line AD since the points of intersection F and G are equally spaced from the line AD. In other words, the angle FAD is equal to the angle GAD. Furthermore, the points G, D, F, E and A are all located on one and the same circle. Consequently, the angle FED is equal to the angle FAD because they both subtend equal arcs on the same circle. In the same manner, it can be established that the angle GED is equal to the angle GAD.

It must now be proved that the angles v are all equal to establish the identity of the angles v with the predetermined required friction angle which was first plotted as the angle between line AD and the vertical diameter yy of the drum. Inasmuch as the line E-D is vertical, it is clear that the angle EDA is equal to the angle v, which is the original friction angle. Therefore, due to the fact that the angles EPA and EGA subtend the same are as the angle EDA, they are equal to the angle EDA.

These relations being established, it can now be shown that the lines of force for the total roller reaction must be defined by the two lines FE and GE and that the two reactions are of equal value. In order to establish this, I must show that the lines of force meet in some point on the vertical line E-D through the center of gravity C.

Furthermore, in order to establish the fact that the tangential projections of all roller reactions are equal and turn in the same direction of rotation and that the hori* zontal projection of the roller reaction from one. side must equal the horizontal projection of roller reaction from the other side, I must prove that the actual intersection point of the two lines of reaction is the point marked E on the drawing where the vertical line through the center of gravity C intersects the horizontal diameter In this particular point, I have:

Projection of reaction F on rail tangent equals F X sine v.

Projection of reaction G on rail tangent equals G sine v.

By the use of the above formulae it is proved that the tangential projections of all roller reactions are equal and turn in the same direction of rotation.

The horizontal projections of F and G will be F X sine a and G X sine a which establishes that the horizontal projection of the roller reaction from one side equals the horizontal projection of the roller reaction from the other side of the drum when the reaction F is equal to the reaction G.

By establishing that the tangential projections of the roller reactions are equal and turn in the same direction of rotation and that the horizontal projection of the roller reaction from one side of the drum equals the horizontal projection of the roller reaction from the other side of the drum, I have proved that the friction angles of the actual applied reaction forces at the rollers are equal for all rollers and that the total pressure load on each of the rollers is equal.

I thus provide by my invention a drive and support system for a rotatable drum in which the leading rollers are positioned asymmetrically with respect to the trailing rollers to equally distribute the pressure therebetween and to permit said rollers to distribute their tractive efforts equally to the girth tracks encompassing the drum.

Although I have shown and described a preferred embodirnent of my invention for the purpose of illustrating the manner of construction and mode of operation thereof, it is obvious that changes may be made in the specific details of the invention and I, therefore, do not intend to be limited thereto but prefer, rather, to be afforded the full scope of the following claim.

I claim as my invention:

In a new and improved rotary grinding mill comprising: a substantially cylindrical, rotatable drum having a plane internal circumference and a generally horizontal axis, said drum being adapted to hold a mass of material being ground and a plurality of objects capable of grinding said material during rotation of said drum; a plurality of girth tracks secured to said drum so as to encompass said drum, said tracks having a substantially smooth outer engageable surface, each of said girth tracks being located in a plane perpendicular to said axis of said drum; a leading drive and support roller engaging each of said girth tracks, each of said leading drive and support rollers having a substantially smooth surface engaging one of said girth tracks, and each of said leading drive and support rollers having a horizontal axis; a trailing drive and support roller engaging each of said girth tracks, each of said trailing drive and support rollers having a sub stantially smooth surface engaging one of said girth tracks, said leading drive and support rollers and said trailing drive and support rollers having equal diameters, and each of said trailing drive and support rollers having a horizontal axis, said axis of said trailing drive and support rollers being vertically spaced lower than said axis of said leading drive and support rollers; and individual power means for said leading drive and support rollers and said trailing drive and support rollers, said individual power means being of equal power thereby delivering equal driving forces to said girth tracks at the peripheries of each roller during normal uniform speed operation.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Weston June 8, 1954 10 FOREIGN PATENTS Great Britain of 1899 Sweden Apr. 23, 1945 Germany June 21, 1911 France Sept. 14, 1931 OTHER REFERENCES The Theory of the Tube Mill, by H. A. White, from the Journal of the Chemical Metallurgical and Mining 10 Society of South Africa, May 1905.

Pages 290305.

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